(a) Field of the Invention
The present invention relates to a method of polycrystallization, a method of manufacturing a thin film transistor, and a laser irradiation device therefor.
(b) Description of the Related Art
In general, a liquid crystal display (“LCD”) includes two panels with electrodes and a liquid crystal layer interposed therebetween. The two panels are combined with a sealant for sealing the liquid crystal layer, which is printed around the edges of the panels. The panels are supported by spacers distributed therebetween.
This LCD displays desired images by applying electric field using the electrodes to the liquid crystal layer with dielectric anisotropy and adjusting the strength of the electric field to control the amount of light passing through the panels. In this case, thin film transistors (TFTs) are used for controlling signals transmitted to the electrodes.
The most commonly used TFTs for an LCD adapts amorphous silicon as a semiconductor layer.
An amorphous silicon TFT has mobility of about 0.5 to 1 cm2/Vsec, which is suitable for a switching element of an LCD. However, it is not sufficient for directly forming a driving circuit on an LCD panel.
In order to overcome such a problem, a TFT LCD including polysilicon with electron mobility of 20 to 150 cm2/Vsec has been developed. The relatively high electron mobility polysilicon TFT enables to implement a chip in glass technique that a display panel embeds its driving circuits.
Techniques for obtaining polycrystalline silicon thin film include a deposition technique depositing polycrystalline silicon directly on a substrate at high temperature, a solid phase crystallization technique depositing amorphous silicon and crystallizing at high temperature of about 600° C., a technique depositing amorphous silicon and crystallizing by laser, and so forth. However, since those techniques require a high temperature process, it is not proper for application of glass substrates for LCDs. Also, they have a disadvantage that electrical characteristics are not uniform between TFTs due to non-uniform grain boundaries.
To solve these problems, a sequential lateral solidification process capable of adjusting the distribution of the grain boundaries has been developed. The process is based on the fact that the grains of polysilicon at the boundary between a liquid phase region exposed to laser beam and a solid phase region not exposed to laser beam grow in a direction perpendicular to the boundary surface. A mask having a slit pattern is provided, and a laser beam passes through transmittance areas of the mask to completely melt amorphous silicon, thereby producing liquid phase regions arranged in a slit pattern. Thereafter, the melted amorphous silicon cools down to be crystallized, and the crystal growth starts from the boundaries of the solid phase regions not exposed to the laser beam, and proceeds in the directions perpendicular to the boundary surface. The grains stop growing when they encounter each other at the center of the liquid phase region. The sequential lateral solidification process is performed with moving a die, which mounts a panel including the amorphous silicon film thereon, in a horizontal direction when irradiating the laser beam and such a scanning step is repeated along the horizontal direction to cover all areas of the panel.
The laser beam irradiation in the sequential lateral solidification process is made through a projection lens. At this time, the laser beam may be precisely focused on desired locations.
However, the focus of the laser beam varies depending on the temperature of the projection lens such that the crystallization of the polysilicon layer for the thin film transistor is non-uniform. In order to solve such a problem, it is most important to develop a technique of keeping the temperature of the projection lens constant when irradiating the laser beam.